In elevator systems, movement of the elevator car is permitted only when the doors of the elevator car and of the shaft are closed and the other conditions required to ensure the safety of passengers are fulfilled. In elevator systems in which the electric motor that moves the elevator car is controlled by means of a frequency converter according to the commands of the control unit of the elevator, safety is typically ensured with a safety circuit. The safety circuit can be implemented e.g. such that switches, which are connected to each other in series, are placed in the points that are essential from the standpoint of safety. The electricity supply of the elevator motor and opening of the holding brakes are only permitted if all the switches of the safety circuit are closed.
The status of the safety circuit typically controls the electricity supply appliance and the brakes of the elevator by means of electromechanical contactors such that when the safety circuit is open the contactors enabling the electricity supply of the motor and of the electromechanical brakes are also open. The contactors close only when the safety circuit closes, in which case the elevator car should not be able to move when the safety circuit is open.
The electromechanical contactors, by means of which the electricity supply of the electricity supply appliance and of the brakes of the elevator are controlled, are however susceptible to switching interference, which can lead to a dangerous situation. Furthermore the contactors are wearing devices, the operating life of which is limited, and noise is generated in conjunction with mechanical switchings, which reduces the passenger comfort of the elevator.
It is also possible that the current supply of the coil of the contactor is controlled by means of a processor via a controllable relay. In this case a dangerous situation can arise also e.g. as a consequence of a malfunction of the processor or from an electromagnetic pulse connected to the system, which can short-circuit the transistors of the system.